Sexual dimorphism in mammals is dictated by a Y chromosomal genetic switch that overrides the autonomous program which would lead to the female phenotype. SRY, a single copy gene in the 1A1 interval in the distal short arm of the Y chromosome, has been identified as the master switch that initiates the events leading to testicular differentiation and male development. SRY has been found to be a transcription factor with an HMG- box DNA binding domain, but no readily recognizable transcriptional activation domain. We have discovered high affinity SRY binding sites (SRYe) in the promoters of the Mullerian Inhibiting Substance (MIS) and P450 aromatase genes. We propose that SRY controls sexual dimorphism by acting coordinately on the promoter elements of these sex related genes, MIS, a glycoprotein responsible for regression of the female reproductive ducts, and P450 aromatase, which converts testosterone to estradiol. We propose that activation of MIS and inhibition of aromatase expression will be dictated by how SRY reconfigures DNA, by position and orientation of SRYe sites, and by their mode of interaction with other binding proteins and transcription factors. The molecular mechanisms worked out for SRY will be relevant to other members of the HMG-box transcription factor family. We will confirm the physiologic relevance of the SRY/SRYe interaction in cotransfection experiments. Then we will generate a male stage-specific SRY and MIS-expressing gonadal cell line made immortal by retroviruses harboring the v-myc oncogene, for use in reporter plasmid transfection assays. This novel cell line can be used to identify endogenous elements and factors that work with SRY to regulate expression of MIS, our model sex-specific gene. An additional DNA-binding protein, M2, is tissue, sex, and ontogeny specific in its recognition of an MIS promoter site, M2e, that is adjacent to the SRYe. We have purified the M2 protein to homogeneity, will clone its cDNA for further studies, then determine if M2 is required for expression of MIS. Since SRY has no recognizable transcriptional activating domains, we may find that cooperation between M2, SRY, and other regulators that we define in these studies may be required for expression of MIS. SRY and other transcription factors uncovered in this study will provide new tools which can increase our understanding of sexual differentiation and the mutations leading to intersex abnormalities and may lead to the discovery of new therapeutic vehicles for use in the long-term study of fertility and contraception. We speculate, however, that the most fundamental and important outcome of this study will be the further clarification of the hierarchical cascades of molecular events that lead to sexual dimorphism and proper sexual differentiation.